The present invention relates generally to methods and apparatus for calibrating an array of charge-coupled devices and, more particularly, to methods and apparatus for uniformly illuminating an array of charge-coupled devices during a calibration process.
Many conventional detectors include an array of charge-coupled devices (CCDs) for obtaining precise optical measurements. For example, CCD cameras include an array of CCDs. CCDs are utilized by detectors since the CCDs are capable of measuring light intensity with a very high level of sensitivity over a large dynamic range. In this regard, CCD cameras can measure light intensity with much greater sensitivity than film. In addition, the output provided by a CCD camera can be readily digitized for subsequent processing by a computer or other digital processor.
Due to the sensitivity of the CCDs, detectors that include an array of CCDs are generally utilized in applications that demand high levels of precision and accuracy. One application that demands high levels of precision and accuracy relates to pressure mapping during wind tunnel tests of objects, such as model aircraft, that are coated with pressure-sensitive paint. In this regard, in order to measure the pressures to which an aircraft is subjected during flight, a scaled model aircraft coated with pressure sensitive paint can be placed in a wind tunnel as described by U.S. Pat. No. 5,359,887 to Scott D. Schwab, et al. When exposed to fluorescent lighting, the paint on the model aircraft emits light having an intensity that corresponds to the air pressure to which that portion of the aircraft is subjected. In order to record and measure the intensity of the light emitted by the pressure sensitive paint, a detector, such as a CCD camera, can obtain an image of the model aircraft. These images can thereafter be digitized and analyzed to determine the relative air pressures to which the various portions of the aircraft are subjected.
During this experiment, a CCD detector is utilized to precisely measure variations in light intensity due to the sensitivity of the CCDs. In order to obtain meaningful measurements, however, the array of CCDs must be calibrated so that each individual CCD responds in the same manner, i.e., there should not be any pixel-to-pixel variation in the gain.
Current calibration techniques employ a process called flat field correction. During this process, the array of CCDs is exposed to light having a uniform intensity and the outputs of the CCDs are adjusted such that the adjusted output of each CCD is identical, thereby normalizing any pixel-to-pixel variation in the gain provided by the array of CCDs. Conventionally, this calibration process is performed by mounting the detector containing the array of CCDs to an integration sphere that uniform illuminates the CCDs. Since the integration sphere is generally mounted to an optical bench, the detector must be transported to the optical bench, mounted to the integration sphere and calibrated.
Unfortunately, it has been found that detectors that include arrays of CCDs are not calibrated as often as is desired. While a number of factors contribute to this problem, it is believed that the relative immobility of the optical bench and, in turn, the integration sphere is a primary factor. In this regard, since the optical bench typically resides in a laboratory, a detector that includes an array of CCDs must generally be removed from service and taken from the field to the laboratory in order to calibrate the CCDs. Since the users of the detectors apparently do not wish to expend the additional time and effort required to transport the detector to and from the laboratory, many detectors that include arrays of CCDs are rarely calibrated, thereby reducing the reliability and, in some instances, the quality of the optical measurements obtained by the detectors.
It is therefore an object of the present invention to provide a calibration apparatus that reduces the time and effort needed to calibrate detectors that include arrays of charge-coupled devices.
It is another object of the present invention to provide a portable calibration device.
These and other objects are provided, according to the present invention, by a portable apparatus for uniformly illuminating a detector comprised of a plurality of CCDs during calibration of the respective gains of the CCDs. The illumination apparatus includes a housing, such as a cup-shaped housing, which defines an internal cavity and which opens through a first end. The illumination apparatus also includes a light source and, more preferably, a photoluminescent sheet that is disposed within the internal cavity for uniformly illuminating the plurality of CCDs to thereby permit the CCDs to be calibrated.
The housing is preferably sized such that the illumination apparatus is portable. In addition, the housing is adapted to be mounted to the detector, such as a CCD camera. For example, the portable illumination apparatus can also include a mounting ring, operably connected to the first end of the housing, for mounting the housing to the detector. As such, the illumination apparatus can be readily mounted to a detector, such as a CCD camera, such that the array of CCDs can be calibrated in an efficient manner in the field, i.e., without requiring the detector to be transported to a laboratory for calibration.
In one preferred embodiment in which the light source is a photoluminescent sheet, the photoluminescent sheet is preferably placed between a pair of support sheets. Advantageously, the support sheet that is disposed on the side of photoluminescent sheet that faces the first end of the housing is formed of a material, such as plexiglass, that is at least partially transparent and uniformly diffuse. As such, light emitted by the photoluminescent sheet can pass through the support sheet and uniformly illuminate the plurality of CCDs during the calibration process.
Therefore, the illumination apparatus of the present invention simplifies the process of calibrating a detector that includes an array of CCDs while continuing to provide accurate calibration. In particular, the illumination apparatus is compact and, therefore, portable such that the illumination apparatus can be carried with the detector so as to calibrate the detector in the field. In addition, the illumination apparatus can be easily mounted to the detector in much the same fashion that a camera lens is mounted to a camera, thereby further simplifying the calibration process. Since the illumination apparatus of the present invention simplifies the calibration process and permits calibration to be performed on demand in the field, it is believed that detectors will be calibrated more frequently, thereby improving the quality and reliability of the images captured by the detectors.